1. Technical Field
This invention relates to the elimination of voids formed when material deposited over a surface of a substrate having holes therein, such as for vias, contacts, or lines, fills a top portion of the holes but not a bottom portion thereof, and more particularly, to the elimination of such voids formed during a physical vapor deposition process.
2. Background Art
Physical vapor deposition (PVD) of materials onto a substrate is a vital step in the manufacture of semiconductor devices. For example, this process is often used to deposit electrically conductive materials into high aspect ratio holes (i.e. holes with a depth to width ratio greater than one) forming inter-level vias or contacts within a semiconductor substrate. Traditionally, physical vapor deposition techniques have involved sputtering particles from a sputter target into a PVD processing chamber which is held in a ultra-high vacuum condition. Typically, the sputtered particles have a neutral charge and tend to move in straight line paths within predictable trajectories from the target surface. Most of these trajectories are other than perpendicular to the surface of the substrate. In some applications, a plasma is used to ionize some of the sputtered particles, which are then drawn toward the substrate by an electrically biased substrate pedestal. The biased pedestal imparts a directionality to the ionized atoms, including those traveling non-perpendicular to the substrate, so that they impact the substrate in a predetermined orientation. Ideally, for filling the aforementioned high aspect ratio holes, the direction of the ionized atoms would be perpendicular to the surface of the substrate. However, the aforementioned plasma formed in traditional physical vapor deposition processing devices usually exhibits a relatively low density, and so a very low ionization rate. Consequently, few of the sputtered atoms are actually ionized and most remain neutrally charged. Thus, in either of the above-described sputtering processes, a majority of the sputtered particles are neutrally charged, and do not change direction after leaving the surface of the target. This results in many of the sputtered particles not even reaching the substrate, and those that do impinge on the substrate in random directions. Many of these particles impact in a generally traverse direction to the surface of the substrate and build up along the upper walls of a high aspect ratio hole. This causes the top of the hole to be filled first and close off, leaving a void in the bottom portion. Such a void in the bottom of the hole can make the inter-level via an unreliable electrical contact.
FIG. 1 exemplifies a semiconductor substrate 10 produced according to the aforementioned prior art physical vapor deposition methods. As can be seen in this cross-sectional view, the substrate 10 includes a bottom layer 12 of silicon with an overlying oxide layer 14 (i.e. SiO.sub.2). A high aspect ratio hole 16 exists in the oxide layer 14. In addition, a layer 18 of aluminum (Al) has been deposited over the oxide layer 14. Due to the aforementioned randomly impacting sputtered atoms, the deposition material has built up on the upper walls of the hole 16 and closed it off, resulting in a void 20 in the lower portion of the hole 16.
In the past, attempts at eliminating the void once it has formed have involved heating the substrate and/or subjecting it to a high pressure environment. The heating is intended to soften the deposited material so that it flows into the hole, a process known as "reflow". At elevated temperatures, a metal will creep along a surface and fill any low spots in the film layer within which the metal resides. Although this method can work for many applications, temperatures high enough to flow some deposition materials would also damage the substrate or the structures formed thereon. Accordingly, heating the substrate to eliminate the void is sometimes undesirable. Subjecting the substrate to a high pressure atmosphere mechanically forces the material deposited over the top of the hole into the void space due to the force exerted by the high pressure atmosphere on the material. The material may have been softened by heating to facilitate this forcing. Although this latter method works well for the applications it was intended, some disadvantages exist. For example, the pressures required to force material into the void can sometimes approach 10 kpsi depending on the characteristics of the deposition material and its thickness over the void. A pressurizing device capable of producing pressures as high as 10 kpsi can be quite complex. In addition, the high pressure can in some instances damage the substrate or the structures formed thereon.
There has been an attempt to avoid the use of these high temperatures and pressures by employing ultrasonic vibration during the deposition process. This method involves fastening the substrate to a base, then subjecting the base, and so the substrate, to ultrasonic energy while at the same time sputter depositing aluminum on the substrate. The substrate is also heated, but at a lower temperature than typically used in the previously described processes. Although, this method works well for the application it was intended, there are drawbacks. The process must be carried out in a deposition chamber which has been specifically modified to impart the ultrasonic energy and heat during the actual deposition procedure. It is unknown what effects the ultrasonic energy and relatively low temperature will have on the deposition process or the resulting deposition layer, or whether existing deposition chambers can be retrofitted with the necessary equipment. In addition, it is believed that the addition of the steps required to apply the ultrasonic energy and heat could increase the deposition processing time, thereby reducing substrate throughput.
Therefore, a need exists for a method of eliminating the void left at the bottom of high aspect ratio holes which does not rely on subjecting the substrate to damaging heat or pressure, and which is not performed, in situ, during the deposition process.